There are lots of elements to rendering, and lots of things to slow it down. Various manuals cover in depth, but some quick rules of thumb, in addition to those already mentioned:
Keep light sources to a minimum, i.e. do not try to replicate real world
Avoid line lights and area lights
Avoid complex textures with bump maps, maybe reflectivity maps
Minimize transparencies/reflectivities
Tinker with settings in Custom RW, though Open GL will always be faster...
HTH.
BTW, you're on the bleeding edge with 10.5.6 !!??